The advance into former insurgent strongholds that had largely been calm before the Americans withdrew less than three years ago is spreading fear that Prime Minister Nouri al-Maliki, struggling to hold onto power after indecisive elections, will be unable to stop the Islamic militants as they press closer to Baghdad.

Fighters from the Islamic State of Iraq and the Levant militant group took control Tuesday of much of Iraq’s second-largest city, Mosul, sending an estimated half a million people fleeing from their homes. As in Tikrit, the Sunni militants were able to move in after police and military forces melted away after relatively brief clashes.

The group, which has seized wide swaths of territory, aims to create an Islamic emirate spanning both sides of the Iraq-Syria border. The capture of Mosul – along with the fall of Tikrit and the militants’ earlier seizure of the western city of Fallujah – have undone hard-fought gains against insurgents in the years following the 2003 invasion by U.S.-led forces.

The White House said the security situation has deteriorated over the past 24 hours and that the United States was “deeply concerned” about ISIL’s continued aggression.